Wafer boat rotating apparatus

ABSTRACT

A wafer boat rotating apparatus is disclosed, which includes a boat holding portion for holding both ends of the wafer boat, a vertical rotation drive mechanism for rotating the boat holding portion on a vertical plane, and a horizontal rotation drive mechanism for rotating the boat holding portion on a horizontal plane. A wafer holding member is disposed on the boat holding portion and adapted to keep the wafers held in the wafer boat apart from a bottom portion of the wafer boat by a small distance. Thus, compensation for the difference of coefficients of thermal expansion between the wafers and wafer boat during heat treatment can be made easily.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wafer boat rotating apparatus forrotating a wafer boat holding wafers.

2. Description of the Related Art

Generally, when semiconductor wafers and liquid crystal wafers areproduced, oxide films, metal films, single-crystal films, and so forthare deposited or impurity-diffused on wafer surfaces. As units forperforming such treatments, heat-treating units such as an oxidizingunit, a CVD unit, an epitaxial unit, and a diffusing unit are known.

In such a heat-treating apparatus, for example six lots (namely, 150wafers) which are arranged with an equal pitch are held by a wafer boatmade of heat resisting material which can withstand high temperaturesand does not give rise to impurities at high temperatures. An example ofsuch a heat resisting material is quartz.

Normally, as heat-treating apparatuses, upright type heat-treatingfurnaces have been widely used. In this heat-treating furnace, a waferboat is placed in an upright (vertical) position. The wafer boat istransferred using one of the following two systems. One system isreferred to as a stand-along system. In this system, wafers transferredby an arm to a wafer boat which is placed in a upright position. Thisarm carries one or five wafers at a time. The other system is referredto as batch system. In the batch system, 25 wafers are transferred to awafer boat which is placed in a flat position at a time. After all 150wafers are transferred, the wafer boat is raised.

In the stand-alone system, one heat-treating furnace requires acorresponding transfer unit. On the other hand, in the batch system, onetransfer unit can handle a plurality of (for example, four)heat-treating furnaces. Thus, from the viewpoints of space and costreduction, the batch system is superior to the stand-alone system.Therefore, the batch system is increasingly used.

When the batch system is used, as described above, a wafer boat rotatingapparatus which places in an upright position and a flat position thewafer boat containing wafers is required.

The wafer boat rotating apparatus is disposed between the heat-treatingunit and a wafer transfer unit. The heat-treating unit consists of, forexample, four heat-treating furnaces arranged in series. The wafertransfer unit has a transfer mechanism which transfers for exampletwenty five (25) wafers held in a carrier to a wafer boat which isplaced in a flat position. This wafer transfer mechanism is disclosed infor example U.S. Pat. No. 5,131,799 which is shown in FIGS. 7 and 8thereof. This wafer boat rotating apparatus has a boat holding portionwith a pair of arms which hold both ends of a wafer boat. The boatholding portion is mounted on a rotating shaft of a vertical rotationdrive mechanism which has a motor and so forth. The vertical rotationdrive mechanism rotates the boat holding portion on a vertical plane.The vertical rotation drive mechanism is mounted on a vertical drivemechanism which comprises a ball screw and a motor. The ball screwextends vertically. The motor rotates the ball screw. The vertical drivemechanism vertically moves the boat holding portion. A horizontalrotation drive mechanism is housed in a base and disposed at a lower endportion of the vertical drive mechanism. The horizontal rotation drivemechanism has a motor and so forth. As the vertical drive mechanismrotates on a horizontal plane, the boat holding portion accordinglyrotates on the horizontal plane.

When wafers are transferred, a reception table of the transfer mechanismis raised from a downward position of the carrier on the wafer transferunit, 25 wafers are held upward of the wafer transfer unit. The wafersheld at the upper position are nipped by a wafer nipping member of thetransfer mechanism. Then, the wafer nipping member is horizontally movedto the position above the wafer boat horizontally supported on a boattable of the wafer transfer unit. A reception table is raised from thedownward portion of the wafer boat, and then the 25 wafers are supportedby the reception table. The wafer nipping member is opened and then thereception table is lowered. In this way, the 25 wafers are transferredinto the wafer boat. This operation is repeated for example four times.Thus, a total of 100 wafers are transferred.

After all the wafers have been transferred to the wafer boat, both endsthereof are held by the arms of the boat holding portion. Thereafter,the vertical rotation drive mechanism is rotated by 90° so as to placethe wafer boat in upright position. In addition, the horizontal rotationdrive mechanism is rotated by 180° so as to orient the wafer boat to theheat-treating unit.

By operating the vertical drive mechanism in a predetermined manner, thewafer boat is moved onto the transfer state of the heat-treating unit.This stage is moved to a transfer entrance/exit of a predeterminedheat-treating furnace along a path connected thereto. The wafer boatplaced on the stage is transferred to an elevator of the heat-treatingfurnace by a robot arm. The elevator vertically moves and the wafersheld in the wafer boat are loaded into the heat-treating furnace.

When wafers which have been heat-treated are unloaded from theheat-treating furnace, the above-described operation is performed inreverse order.

The wafer boat comprises two holding plates and four wafer holding rods.Each of the wafer holding rods has a large number of (for example, 100)wafer holding grooves. By each groove, the peripheral portion of a waferis held.

When wafers are transferred to the wafer boat which is placed in a flatposition, a peripheral portion of each wafer comes in contact with abottom portion of a corresponding wafer holding groove of the lowerwafer holding rod. Thus, the wafer is held. In this condition, edgeportions extruding from the holding plate are held and then the waferboat is raised from the flat position to the upright position. Next, thewafers are transferred to the heat-treating furnance and heat-treated inthe furnace.

However, there is a difference in coefficients of thermal expansionbetween quartz (which is a constructional element of the wafer boat) anda wafer. Thus, each wafer should be smooth against the inner wallsurfaces of the wafer holding groove by the amount equivalent to thedifference of the coefficients of thermal expansion. When each wafer isrelatively light (as in a wafer with a diameter of six inches), thedifference of coefficients of thermal expansion is absorbed without aproblem. As the wafer size becomes large (as in eight inches), theweight of the wafer increases. For example, the weight of an eight-inchwafer is twice the weight of a six-inch wafer. Thus, when such largewafers are heat-treated, they do not satisfactorily slide and therebythe difference of coefficients of thermal expansion cannot be absorbed.Therefore, the stress which take place at the peripheral portion of eachwafer becomes very large and thereby a slip dislocation takes place inthe crystalline direction.

To solve such a problem, the shape of the wafer holding grooves may bechanged in various manners. However, so far, the difference ofcoefficients of thermal expansion between wafer boat and wafers has notbeen satisfactorily absorbed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a wafer boat rotatingapparatus for keeping the peripheral portion of a wafer apart from thebottom portion of the wafer holding groove so as to satisfactorilyabsorb the difference of coefficients of thermal expansion, therebypreventing a slip dislocation from taking place.

The present invention is a wafer boat rotating apparatus disposedbetween a wafer transfer unit being adapted to transfer a plurality ofwafers to a wafer boat which is held in a horizontal direction and whichdefines an opening at a bottom portion thereof and a heat-treating unitbeing adapted to heat-treat the wafers held in the wafer boat held invertical direction, the apparatus comprising a boat holding portion forholding both ends of the wafer boat, a vertical rotation drive mechanismfor rotating the boat holding portion on a vertical plane thereof, ahorizontal rotation drive mechanism for rotating the boat holdingportion on a horizontal plane thereof, and a wafer holding memberdisposed on the boat holding portion and being adapted to keep thewafers held in the wafer boat apart from a bottom portion of the waferboat by a small distance.

According to the present invention, when the wafer boat placed in flatposition is held by the boat holding portion, the wafer holding memberdisposed on the boat holding portion keeps each wafer apart from thebottom portion of the wafer boat by a small distance. In this condition,the vertical rotation drive mechanism is operated so that the wafer boatis raised. Thus, the side surface of each wafer is held in the waferboat. In this condition, the horizontal rotation drive mechanism and thevertical drive mechanism are operated so that the wafer boat istransferred to the heat-treating unit. In the heat-treating unit, eachwafer is heat-treated. The difference of coefficients of thermalexpansion between the wafer boat and the wafers which takes place in theheat treatment is absorbed since the wafers satisfactorily slide. Thus,the wafers can be prevented from being stressed.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of a best mode embodiment thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view showing a wafer boat rotating apparatusaccording to an embodiment of the present invention;

FIG. 2 is a perspective view showing a boat holding portion of the waferboat rotating apparatus of FIG. 1;

FIG. 3 is a side view showing the wafer boat where the wafer boat isheld by the boat holding portion;

FIG. 4 is a sectional view showing a wafer holding portion;

FIG. 5 is a partially enlarged view showing a wafer holding rod forexplaining the operation of the present invention;

FIG. 6 is a plan view schematically showing an entire system includingperipheral units for explaining a position at which the wafer boatrotating apparatus is installed;

FIG. 7 is a detailed perspective view showing the wafer boat;

FIG. 8 is a perspective view of FIG. 6;

FIG. 9 is a schematic diagram for explaining a condition where a slipdislocation takes place in a heat treatment in the case that the waferholding portion is not provided;

FIG. 10 is a schematic diagram for explaining a condition where waferscome in contact with corresponding wafer holding grooves of a waferholding rod in the case that the wafer holding portion is not provided;and

FIG. 11 is a detailed schematic diagram showing a wafer lift-up plate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Next, with reference to the accompanying drawings, a wafer boat rotatingapparatus according to an embodiment of the present invention will bedescribed in detail.

FIG. 1 is a perspective view showing a wafer boat rotating apparatusaccording to an embodiment of the present invention. FIG. 2 is aperspective view showing a boat holding portion of the wafer boatrotating apparatus of FIG. 1. FIG. 3 is a side view showing the waferboat where the wafer boat is held by the boat holding portion. FIG. 4 isa sectional view showing a wafer holding portion. FIG. 6 is a plan viewschematically showing an entire system including peripheral units forexplaining a position at which the wafer boat rotating apparatus isinstalled. FIG. 8 is a perspective view of FIG. 6.

In FIGS. 6 and 8, a wafer boat rotating apparatus 50 is disposed betweena heat-treating unit 6 and a wafer transfer unit 14. The heat-treatingunit 6 consists of for example four heat-treating furnaces 4 which aredisposed in series. The wafer transfer unit 14 has a transfer mechanism12 which transfers for example twenty five (25) wafers W held in acarrier 8 to a wafer boat 10 placed in a flat position. Theheat-treating furnaces 4 each have an entrance/exit opening 4a.

As shown in FIG. 1, the wafer boat rotating apparatus 50 comprises aboat holding portion 18, a vertical rotation drive mechanism 22, avertical drive mechanism 26, a horizontal rotation drive mechanism 30,and a wafer holding member 52. The boat holding portion 18 holds edgeportions 47a and 47b (see FIG. 3) disposed on both ends of the waferboat 10. The vertical rotation drive mechanism 22 rotates the boatholding portion 18 on a vertical plane. The vertical drive mechanism 26vertically moves the boat holding portion 18. The horizontal rotatingdrive mechanism 30 rotates the boat holding portion 18 on a horizontalplane. The wafer holding member 52 slightly lifts up the wafers W fromthe wafer boat 10 when the wafer boat 10 is raised in a flat position.

Practically, the vertical rotation drive mechanism 22 is provided with amotor and so forth (not shown). A rotating shaft 20 of the verticalrotation drive mechanism 22 is connected to a longitudinal centerportion of the boat holding portion 18 so that the boat holding portion18 is rotated on a vertical plane.

The vertical drive mechanism 26 has a ball screw 24 and a motor 54. Theball screw 24 extends vertically. The motor 54 rotates the ball screw24. A movable member 56 which moves along the ball screw 24 is connectedto the vertical rotation drive mechanism 22. Thus, the boat holdingportion 18 moves vertically. The vertical drive mechanism 26 is coveredby a housing 58 having a cubic shape.

The horizontal rotation drive mechanism 30 is disposed at a lower endportion of the vertical drive mechanism 26. The horizontal rotationdrive mechanism 30 is housed in a base 28. The horizontal rotation drivemechanism 30 consists of a motor and so forth. As the vertical drivemechanism 26 is rotated on a horizontal plane, the boat holding portion18 is rotated on a horizontal plane.

As shown in FIG. 2, the boat holding portion 18 has a holding brace 60.A longitudinal center portion of the holding brace 60 is connected tothe rotating shaft 20. The holding brace 60 extends nearly in thelongitudinal direction of the wafer boat 10. A pair of Y-letter shapedboat holding arms 62 are disposed at both ends of the holding brace 60in such a way that open portions of the boat holding arms 62 faceupward. These open portions hold the edge portions 47a and 47b disposedon both ends of the wafer boat 10. A holding arm 64 extends horizontallyfrom a lower portion of the holding brace 60 to a portion below thewafer boat 10. The holding arm 64 is made of for example stainless steelso as to have a predetermined rigidity which can withstand the weight ofthe wafers W.

The holding arm 64 has a pair of wafer lift-up plates 66 spaced apart bya predetermined distance along the longitudinal direction of the holdingbrace 60 (namely, in the longitudinal direction of the wafer boat 10).The holding arm 64 and the pair of wafer lift-up plates 66 compose thewafer holding member 52. A space which is open upward and downward isformed between the pair of wafer lift-up plates 66. A reception table 82is provided within the space 80 and is vertically driven by a cylinder84. The 25 wafers W are vertically held by the reception table 82. Thecenter between the pair of wafer lift-up plates 66 accords with thecenter between the pair of boat holding arms 62 (see FIG. 2). When thewafer boat 10 is held by the boat holding portion 18, the wafers W arelifted up by the pair of wafer lift-up plates 66. Thus, the wafers W arekept apart from a lower wafer holding rod 40B by a small distance L1,(for example, 0.5 to 1.0 mm) (see FIG. 4).

Thus, the height L2 of the wafer lift-up plates 66 is determined so thatwhen the wafers W are transferred to the wafer boat 10 placed in a flatposition (horizontally), the upper end of the wafer lift-up plates arehigher than the lower periphery 68 of the wafers W by a small distance(for example, 0.5 to 1.0 mm). When the wafers W having a diameter ofeight inches are used, the distance L3 between the wafer lift-up plates66 is determined to approximately 108 mm. At this time, the height L2 ofthe wafer lift-up plate 66 is determined to be approximately 28 mm sothat the lower periphery of the wafers W does not come in contact withthe holding arm 64.

As shown in FIGS. 3, 5, and 7, the wafer boat 10 comprises a pair ofholding plates 38 and four wafer holding rods 40A and 40B. The fourwafer holding rods 40A and 40B extend between the pair of holding plate38. The wafer holding rods 40A and 40B each has a large number of (forexample, 150) wafer holding grooves 42, each holding a periphery portionof each of the wafers W.

As described above, when the wafer boat 10 placed in a flat position(namely, horizontally) is held by the boat holding portion 18, thewafers W are lifted up by the pair of wafer lift-up plates 66. When thewafer boat 10 is raised in a upright position, the wafers W are held byinner wall surfaces 42a of the corresponding wafer holding grooves 42(see FIG. 5). In addition, as described above, the pair of holdingplates 38 of the wafer boat 10 have the edge portions 47a and 47b whichare held by the pair of Y-letter shaped boat holding arms 62 and 62,respectively. The diameter of the edge portion 47b is larger than thediameter of the edge portion 47a. The edge portion 47b has a flange 48.Reinforcement members 85 are arranged between the two wafer holding rods40A and between the two wafer holding rods 40B in a directionperpendicular to the longitudinal direction of the wafer boat 10.

As shown in FIG. 4, the thickness L5 of the pair of lift-up plates 66 isdetermined to approximately 4 mm. Respective upper end portions of thepair of wafer lift-up plates 66 face to each other and inclinedownwardly in an inner direction corresponding to a circular shape ofthe wafers W. As shown in FIG. 11, the wafer lift-up plates 66 each hasa contact portion 66a. The contact portion 66a is made of material whichhas a high temperature resistance and stability so as to prevent thewafers W from being contaminated with metals. Examples of such materialare Teflon (trade name) and quartz. The contact portion 66a is held by abody 66b made of stainless steel.

As shown in FIG. 3, concave portions 70 are formed in the wafer lift-upplates 66. The concave portions 70 prevent the wafer lift-up plates 66from interfering with the reinforcement members 85. A reinforcementplate 72 is disposed in the vicinity of the concave portions. Thereinforcement plate 72 connects the pair of wafer lift-up plates 66.

Next the operation of the embodiment will be described.

As shown in FIG. 6, all the 25 wafers W held in the carrier 8 aretransferred to the wafer boat 10 placed in flat position (horizontally)on the boat table 14a of the wafer transfer unit 14 by the transfermechanism 12 at a time. By repeating this transfer operation for examplesix times, 150 wafers W are transferred to the wafer boat 10. In thisembodiment, the wafers W transferred from the transfer unit 12 arereceived by a reception table 82. The reception table 82 is verticallymoved by the cylinder 84 in the space 80 defined by the pair of waferlift-up plates 66. In this condition, as shown in FIGS. 9 and 10, thewafers W are held in the wafer holding grooves 42 defined on the waferholding rods 40A and 40B. Thus, outer peripheral portions 46 of thewafers W come in contact with bottom portions 44 of the two lower waferholding rods 40B.

As the wafer boat rotating unit 50 is operated, the wafer boat 10 israised by the boat holding portion 18 which is placed in a flat positionas shown in FIG. 3. At this time, the edge portions 47a and 47b whichprotrude from the holding plates 38 and 38 are held by the Y-lettershaped boat holding arms 62, respectively. Thus, the entire wafer boat10 is horizontally raised. At this time, since the holding arm 64 hasthe pair of wafer lift-up plates 66, the upper end portions of the waferlift-up plates 66 come in contact with the lower peripheral portion 46of each of the wafers W, thereby lifting up the wafers W by a slightdistance L1, for example, 0.5 to 1.0 mm. Consequently, as shown in FIGS.4 and 5, the lower peripheral portion 46 of each of the wafers W is keptapart from the bottom portion 44 of the corresponding holding groove 42of each of the two lower wafer holding rods 40B nearly by the distanceL1. It should be noted that as with the two lower wafer holding rods40B, the peripheral portion 46 of each of the wafers W may be kept apartfrom the bottom portion 44 of the corresponding holding groove 42 ofeach of the two upper wafer holding rods 40A by the distance L1.

As described above, when the wafer boat 10 placed in flat position(horizontally) is raised by the boat holding portion 18, the wafers Ware lifted up from the wafer boat 10 by the small distance L1. Thus, theperipheral portion 46 of each of the wafers W is kept apart from thecorresponding groove bottom portion 44.

In this condition, by rotating the vertical rotation drive mechanism 22by 90° at a low speed, the wafer boat 10 is raised in an uprightposition. Next, by rotating the horizontal rotation drive mechanism 30by 180° , the wafer boat 10 is oriented to the heat-treating unit 6.Thus, each of the wafers W is held on the inner wall surface 42a of thewafer holding groove 42 of each of the four wafer holding rods 40A and40B.

As shown in FIGS. 6 and 8, by operating the vertical drive mechanism 26in a predetermined manner, the wafer boat 10 is lowered. The wafer boat10 which is placed in upright position is moved to a table 32a of thetransfer stage 32. The stage 32 moves to an entrance/exit opening of apredetermined heat-treating furnace 4 of the heat-treating unit 6 alonga transfer path 34. The wafer boat 10 placed on the table 32a of thestage 32 is transferred to an elevator (not shown) of the heat-treatingfurnace 4 by a robot arm 36 disposed at each heat-treating furnace 4.The elevator upwardly moves and thereby the wafer boat 10 is loaded tothe heat-treating furnace 4.

The wafers W loaded to the heat-treating furnace 4 are heat-treatedcorresponding to predetermined process conditions. In this heattreatment, the coefficient of thermal expansion of quartz (5.4×10⁻⁷ /°C.) forming the wafer boat 10 is smaller than the coefficient of thermalexpansion of silicon (4.8×10⁻⁶ /° C.) forming the wafers W, resulting ina difference of the thermal expansion between the wafer boat 10 and thewafers W. As described above, since the peripheral portion 46 of each ofthe wafers W is kept apart from the corresponding groove bottom portion44 of each of the wafer holding rods 40A and 40B, each of the wafers Wequally slides on the inner wall surface (contacting surfaces) 42a ofthe corresponding wafer holding groove 42 of each of the upper and lowerwafer holding rods 40a and 40B. Thus, the groove bottom portion 44 ofeach wafer holding groove 42 does not apply any excessive stress to theperipheral portion 46 of each of the wafers W. As a result, the wafers Wcan be securely prevented from slip-dislocating.

In other words, as shown in FIGS. 9 and 10, unless the wafer lift-upplates 66 are provided, each of the wafers W in the wafer boat 10 placedin flat position comes in contact with the bottom portion 44 of thecorresponding wafer holding groove 42 of each of the lower wafer holdingrods 40B due to the dead weight of the wafers W. When the wafer boat 10is raised in an upright position and then is heated at approximately1000° C., the wafers W having a diameter of eight inches expand byapproximately 1 mm, whereas the wafer boat 10 expands by approximately0.1 mm. At this time, due to the difference of the thermal expansion ofthe wafer boat 10 and the wafers W, a slip dislocation 87 takes place inthe wafers W in the vicinity of the lower wafer holding rods 40B.

However, according to the present invention, since the peripheralportion 46 of each of the wafers W is out of contact with the bottomportion of the corresponding holding groove of each of the wafer holdingrods, each of the wafers W slides on the inner wall surface 42a of thecorresponding wafer holding groove. Thus, the difference of the thermalexpansion which take place between the wafer boat 10 and the wafers Wcan be easily absorbed. After the wafers W have been heat-treated, theyare unloaded from the heat-treating furnace by performing theabove-described operation in reverse order.

In the above-described embodiment, two wafer lift-up plates 66 wereused. However, it should be noted that the number of the wafer lift-upplates 66 is not limited to two. Instead, one wafer lift-up plate may bedisposed just below the center portion of the wafers W (the centerportion of the pair of boat holding arms 62). In this case, the lift-upplate 66 may have a predetermined thickness.

The present invention may be applied to various heat-treating units suchas an oxidizing unit, a CVD unit, an epitaxial unit, and a diffusingunit.

As described above, according to the wafer boat rotating apparatus ofthe present invention, the following excellent effects may beaccomplished.

The wafer boat 10 placed in flat position can be raised while the wafersW are out of contact with the wafer boat 10. Thus, the peripheralportion 46 of each of the wafers can be kept apart from thecorresponding groove bottom portion 44 of each of the wafer holdingrods.

Therefore, the wafers W can be heat-treated without an occurrence ofexcessive stress caused by a difference of the thermal expansions of thewafers and the wafer boat. As a result, the wafers W can be preventedfrom slip dislocating 87, thereby increasing the yield of the wafers W.

Although the present invention has been shown and described with respectto a best mode embodiment thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions, and additions in the form and detail thereof may be madetherein without departing from the spirit and scope of the presentinvention.

What is claimed is:
 1. A wafer boat rotating apparatus disposablebetween a wafer transfer unit adapted to transfer a plurality of wafersto a wafer boat having an opening at a bottom portion thereof, whichboat is held in a horizontal direction, and a heat-treating unit adaptedto heat-treat wafers held in the wafer boat in a vertical direction,said apparatus comprising:a boat holding portion for holding both endsof a wafer boat; a vertical rotation drive mechanism for rotating saidboat holding portion on a vertical plane; a horizontal rotation drivemechanism for rotating said boat holding portion on a horizontal plane;and a wafer holding member disposed on said boat holding portion andbeing adapted to keep wafers held in the wafer boat apart from a bottomportion of the wafer boat by a small distance.
 2. The wafer boatrotating apparatus as set forth in claim 1,wherein said boat holdingportion has a holding brace extending in a longitudinal direction of thewafer boat, and wherein said wafer holding member comprises at least onewafer lift-up plate disposed on said holding member and being in contactwith a lower end of each wafer.
 3. The wafer boat rotating apparatus asset forth in claim 2,wherein said wafer holding member comprises onewafer lift-up plate.
 4. The wafer boat rotating apparatus as set forthin claim 2,wherein said wafer holding member comprises a pair of waferlift-up plates.
 5. The wafer boat rotating apparatus as set forth inclaim 4,wherein said pair of wafer lift-up plates form a space which isopen upward and downwardly.
 6. The wafer boat rotating apparatus as setforth in claim 2,wherein said wafer lift-up plate has a quartz contactportion adapted to be in contact with a lower end of each wafer.
 7. Thewafer boat rotating apparatus as set forth in claim 2,wherein the waferboat has at least one reinforcement member disposed in a directionperpendicular to the longitudinal direction thereof, and wherein saidwafer lift-up plate has at least one concave portion for accepting thereinforcement member.
 8. The wafer boat rotating apparatus as set forthin claim 2,wherein said boat holding portion has a pair of boat holdingarms disposed on said holding brace and adapted to hold both ends of thewafer boat, and wherein a center of said wafer lift-up plate correspondsto a center of said boat holding arms.
 9. The wafer boat rotatingapparatus as set forth in claim 2,wherein the wafer boat comprises apair of holding plates and a plurality of wafer holding rods extendingin the longitudinal direction of the wafer boat between the holdingplates, the wafer holding rods having a plurality of wafer holdinggrooves for holding wafers, and wherein said wafer lift-up plate isadapted to keep wafers apart from a bottom portion of a correspondingwafer holding groove of at least a lowest one of the wafer holding rods.10. The wafer boat rotating apparatus as set forth in claim 9,whereinsaid wafer lift-up plate is adapted to keep wafers apart from the bottomportion of the corresponding wafer holding groove of each of the waferholding rods.
 11. The wafer boat rotating apparatus as set forth inclaim 10,wherein said wafer lift-up plate is adapted to keep wafersequally apart from the bottom portion of the corresponding wafer holdinggroove of each of the wafer holding rods.
 12. The wafer boat rotatingapparatus as set forth in claim 10,wherein held wafers have a diameterof eight inches, and wherein said wafer holding member is adapted tolift up wafers from the wafer boat by 0.5 to 1.0 mm.